How and to what degree does genotypic variation influences age- specific human mortality rates at older age-specific human mortality rates at older ages? The overarching goal of this project is to use data on the life spans of Danish twins and of the general Danish population to address this convergent question in demography and genetics. Six specific aims follow from this goal: Aim 1: Complete follow-up of the like and unlike sexed pairs in the Danish Twin Register through 1992, and then computerize, verify, publish, and archive this data. Aim 2: Compile, verify, computerize, publish, and archive Danish national population and death counts by sex, single year of age, and single year of time from 1870 through 1992. Aim 3: Determine for the Danish data whether the age-specific mortality rates of MZ (monozygotic) twins, like-sexed and unliked- sexed DZ (dizygotic) twins, and unrelated individuals are similar or whether there are specific effects of debilitation or selection due to prematurity, low birth weight, and high twin mortality in infancy. Aim 4: Calculate heritability estimates and concordance comparisons to determine the impact of genotypic variation on variation in the lifespans of Danish twins. Aim 5: Determine, using maximum likelihood estimation, the heterogeneity in genetically-endowed frailty of Danish MZ and DZ twins and the Danish population in general. Aim 6: Determine, using maximum likelihood estimation, whether models and senescent, genetically-programmed death around age 85+- 7) fit the Danish data better than models with only premature death at observable ages. That is, test the hypothesis that there is a genetic cap on individuals' lifespans around age 85+-7.